There are several well-known methods for trying to grapple a measuring sonde whose handling is no longer possible through the cable, but none allows measurements to be continued at the place where they have been stopped following the immobilization of the sonde or its sticking.
A first method is common to all operations for fishing tubular parts stuck or lost in a wellbore. The well is first cleared of the measuring cable so that it does not hinder later operations. To that effect, the cable is pulled until it breaks at the brittle point which is located on the fixing device on the upper part of the sonde. After taking up the cable by means of its winch, a fishing string mainly consisting of an "overshot", as it is commonly called in the profession, adapted to clutch the top of the sonde, is run down into the well. The other components are conventionally pipes and drill collars. In this method, the difficulty consists of covering the sonde with the overshot in the absence of guiding and while groping along from the surface. This operation may actually only succeed at a slight depth and in instances where the drill hole is well calibrated and where the axis of the tubular to be fished is almost parallel to the axis of the well. Concerning measuring sondes, most of them have a small diameter with respect to the hole and these conditions are scarcely present, except in small-diameter boreholes.
The most common method is called the "cut and thread" method. It consists of cutting the cable at the level of the derrick floor without dropping the part of the cable linked with the sonde into the well. Thus, the two ends of the cut cable are provided with two half-elements constituting a quick coupling. Assemblage of the overshot and of the first drill collars is started in the derrick. The end of the cable connected to the winch is passed through these first elements when they hang on the pipe hook. The two ends of the cable are connected through the quick coupling. The cable may then be maintained taut by its winch while the overshot and the first pipes are lowered around the latter into the well. After hanging them onto the rotary table, the cable is maintained before the quick coupling is opened so as to pass the end of the cable connected to the winch through new tubular elements assembled and hung on the pipe hook, as previously. The lowering maneuver is continued by repeating this operation until the overshot, guided by the coaxial cable, covers and clutches the top of the sonde. The fishing operation is ended, as in the previous method, after the cable has been broken.
This operation is long because passing the cable through each assembled length of tubular elements causes a waste of time which is relatively considerable in relation to the usual maneuver time. In case of sticking in a borehole, it is an accepted fact that speed is a preponderant factor for the success of the fishing operation.
None of the two methods described above allows measurements to be achieved or continued with the sonde in said well.
The method in accordance with the present invention reduces the maneuver time by limiting the number of operations for running the cable through the length of tubular elements, by using advantageously a side-entry sub.
With the "cut and thread method", the sonde is never connected electrically to the surface and it has never been attempted to keep the use of the sensors of the sonde once the latter is stuck. In fact, the use of a quick coupling including sealed connections for the conductors is of no interest here since the cable will be broken after the sonde has been clutched. Furthermore, the cable being coaxial to the string of tubular elements over its total length, it is not possible to move the sonde while keeping the entire cable continuity.
The method of the invention also has the advantage of allowing measurements to be continued when the sonde has been clutched, be it towards the bottom of the well or higher up towards the surface. The measuring operation, which has been interrupted or made impossible by the immobilization of the sonde, will not be totally missed since it is now possible, with the present method, to carry out the total or at least part of the measuring program.
Besides the main advantage cited above, the invention provides a means for knowing precisely the moment of contact of the grappling sub with the head of the sonde, then for checking the holding back of the sonde by said sub. In fact, the sonde is completely operational since it is connected mechanically and electrically to the surface installation, as at the beginning of the operation. By means of sensors and through the transmission of the signals towards the surface, the operator may control that the displacement of the string makes the sonde move identically. This advantage guarantees not only that further measurements will be possible, but also that grappling of the sonde will succeed, unlike prior methods which provide no reliable information on the quality of the grappling of the sonde, which accounts for the relatively high failure rate in the most difficult cases.
Another method called "side door" method may also illustrate the prior art. It consists of using a special overshot having a lateral opening allowing the measuring cable to be passed outside the fishing string. The cable needs not be cut. The string may then be lowered in a conventional way. The overshot is guided onto the head of the sonde as in the "cut and thread" method, then the operation is continued according to the same methodology. This "side door" method is not used for wells deeper than 1,000 meters because it involves high risks of damage of the cable upon lowering of the string towards the well bottom, and in case of cable breakage, the absence of guiding of the overshot most often compromises recovery of the sonde. In fact, when the overshot gets close to the head of the immobilized or stuck sonde, the well will, by that fact, give rise to considerable friction on the end of the string outside which the cable is located and is therefore very vulnerable. Moreover, the mechanical actions necessary to grapple the sonde are most often exceed the strength of conventional cables. In order to make the limited use of this method quite clear, the following recommendation, given to sonde fishing operators, may be cited: "the side door method should not be used to fish tools in open holes, but rather to fish tools stuck at the shoe of a casing string". But when measuring tools are at this level, measurements are generally finished.